A Whelp is a 1 / 1 Minion. Should it find it's way into a player's hand, it costs 1 Mana Crystal to play.

Uses and Tactics

6 damage at the cost of 4 mana allows Leeroy to act like a Mage's Fireball that anyone can use with a couple of downfalls. First, it's a minion meaning it's affected by minions with Taunt and second, Leeroy spawns two 1/1 Whelps for the opponent forcing the player in most situations to only use Leeroy as a finisher. However, there are also upsides to being a minion, such as the ability to be buffed by many class specific spells that change Leeroy from a mere Fireball to a devastating game ender.

Combos and Syngeries

Rogues most famously use Leeroy as a devastating almost 1 turn killthat requires a lot of cards, a lot of setting up and turn 10 to actually accomplish. One such turn 10 combo is:

Play Leeroy Jenkins to deal 6 damage. [6 mana left, 6 damage dealt]

Cast Shadowstep on Leeroy Jenkins to return him to your hands and reduce his cost from 4 to 2 mana.

Add additional Soulfire(s) to increase damage to 18 or 22 depending on luck or hand at no additional mana cost.

Counters

Rogue: If they are barely playing any minions other than SI:7 Agent or if you see a Gadgetzan Auctioneer, most likely they are trying to do what's called cycling through their deck which is the act of playing a spell to deal damage or remove threats off the board so they can get another piece of their combo. Mainly trading off and stalling until they have all the pieces and enough mana to kill you in 1 turn with.

Shaman: You can't really tell if Shamans are going for this combo, the only way really to know is to remember how many Rockbiters or Lightning Bolts they use and subtract that from the total of damage they potentially could deal. Each Rockbiter Weapon removes 6 damage from their killing combo and each Lightning Bolt reduces the potency as well. Use this sort of measurement to know how much health you need to have to be safe. Counting their cards is also important since the full combo requires 6 cards, if they don't have that many you are pretty safe.

Taunts. If you really can't deal with attacking them faster, the next best thing is to load up on minions with Taunt or minions that give other minions Taunt. Most classes have ways to break through 3-4 taunts, so you would honestly need the ability to bring up 5+ taunts reliably to impede strategies involving Leeroy as a finisher, in which case their strategy just fails miserably, however having lots of taunts works against you in other match-ups so there is a risk involved that allows Leeroy Jenkin finishers to be a viable strategy.